1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly to an active matrix LCD rearranging the layout of its common electrode and modifying a corresponding driving method.
2. Description of the Related Art
The image quality of LCDs deteriorates due to a flicker phenomenon, which is directly relative to the sensitivity of the naked eye. Thin film transistor LCDs (TFT-LCDs) and super twisted nematic LCDs (STN-LCDs) are now generally used for display apparatuses. Unfortunately, both of them also have flicker problems. In most cases, to avoid flicker images, LCDs must be driven in an AC electrical field, because polarity inversion is needed. In LC cells, we find that flicker is mainly caused by the mobile ion charges, even though a higher frequency AC applied to the LC cells can reduce the flicker phenomenon. But power consumption is dependent on the frequency of the AC electrical field. On the other hand, due to a stray capacitor effect, the center level of driving signals shifts between two consecutive frame periods, so the amplitudes of the driving signals are different between the positive polarity and the negative polarity of the LC cells. Therefore, the flicker problem becomes worse.
FIG. 1 is an equivalent circuit diagram of a conventional LCD. The LCD 10 has a plurality of pixels 16 formed by a plurality of parallel data lines 13 perpendicularly crossing a plurality of parallel scanning lines 14. Each of the pixels 16 further includes a thin film transistor (TFT) 161 and an LC capacitor 162 that controls the rotation directions of LC molecules. The data lines 13 and scanning lines 14 respectively transmit driving signals generating from a data driver module 11 and scanning driver module 12, and the driving signals can drive each pixel 16 to have a proper gradation on its color. All the LC capacitors 162 electrically shorting to one of the scanning lines 14 are together electrically connected to a common electrode 151, and all the common electrodes 151 short to a principal common electrode 15 whose potential is driven by a modulation signal source 17.
Generally speaking, in the duration of polarity inversion, the potential of the common electrode 151 can synchronously vary with the variation of the potential of pixel electrodes so as to reduce the operating range of the potential for the pixel electrodes. For example, a 15 inch LCD having 1024×768 pixels execute polarity inversion by row inversion. The principal common electrode 15 needs to have its potential modulated once after each row of the pixels is scanned. We can assume that a vertical scanning frequency is 60 Hz, and the modulation signal source 17 must have a potential modulation frequency around 768×60=46,080 Hz. However, all the common electrodes 151 on the display also have to vary their potentials synchronously with the modulation frequency, thus too much electrical power would be wasted.